I think you should talk to your sister first. Tell her how you feel about her having a boyfriend without your parents knowing about it. Ask her what she is planning to do: Tell your parents or hide the boyfriend forever. I believe that is not good choice to hide a boyfriend from parents.
I was just at home and I heard my sister's bed moving. My sister was gasping and doing other weird sounds. Her boyfriend is in her room and my parents are not home. I don't know what to do Help please my parents don't even know she has a boyfriend?
I think you should go to your sister and ask her to share the matter! If you just go and tell your parents you're trying to be an enemy. So tell her or ask her about it and tell her it will be only between you two only and advise her try to be a brother!